Pretimed three-lobed rotary pump



1963 H. M. TOMASKO ET AL 3,106,166

PRETIMED THREE-LOBED ROTARY PUMP Filed Jan. 20, 1960 s Sheets-Sheet 2 F--/Z0 INVENTOR [HZ up. I? flew) A. /7/'// Howard M Tamasko 1963 H. M.TOMASKO ETAL 3,105,166

PRETIMED THREE-LOBED ROTARY PUMP Filed Jan. 20, 1960 3 Sheets-Sheet 514% /Zfif n 150,454 146- pg 1/ 149 144-1 152 153 1 l i :5 i 8 a;

H 0 7 INVENTOR 52mg, ATTORNEY United States Patent Ofilice EddbdbtiPatented Get. 8, 1963 3,196,166 PRETHWED THREE-LQBED RQTARY PUMP HowardM. Toznasho and Robert A. Hiii, Minneapolis, Minn, assignors to WaterousCompany, St. Paul, MIXHL, a corporation of Minnesota Filed Jan. 20,1960, Ser. No. 3,551 9 Cllaims. (Ci. Eli-12b) This invention relates toan improvement in pre-tirned three lobed rotary pump and method offorming the same and deals particularly with a pump of the typedescribed having gears pre-set in fixed relation to the rotor lobes.

In the production of rotary pumps, considerable time and effort isrequired to properly locate the gears on the shafts. Due to the factthat the lobes on the pump interengage with but slight clearancetherebetween, it is necessary to position the intermeshing gears on therotor shafts with extreme accuracy. In the past, it has been usualpractice to mount the rotors on suitable fixtures, to key the gear onone of the shafts, and to then key the intermeshing gear to the othershaft through the use of a split key or some other arrangement tocompensate for variations in the location of the keyway in the shaft andthe location of the keyway in the gear. Properly timing the gearsrelative to the rotors has usually required from six to twenty hours ofmanual labor. It is an object of the present invention to provide a pumpof the type described having the gears properly related to the rotorsbefore the pump is assembled.

A feature of the present invention lies in the provision of a rotarypump having rotors provided with a predetermined number of lobes, and inproviding intermeshing gears on the rotor shafts which are pre-set sothat upon assembly, the lobes are properly ti-rned. This requires acertain relationship between the rotors and the gears, which will bedescribed.

As will be understood, the lobes of one rotor are separated by valleysto accommodate the lobe of the cooperating rotor. As long as the gearson both rotor shafts have the same number of gear teeth, the two rotorswill operate in unison. We have found, however, that by properlychoosing the gears, and by securing each gear to the rotor shaft so thatthe center of a gear tooth is directly aligned with the center of therotor lobe, the rotors and their gears. may be pre-timed. Obviously thesame result can be accomplished by-centering a gear tooth space (midwaybetween the gear teeth) with the center of a gear lobe. To do this twothings are necessary. In the first place, it is necessary that thenumber of teeth on the gears be a multiple of the number of lobes.Secondly, it is necessary that, midway of the angular distance betweenthe lobes, the gear must have a gear tooth space, when the gear teethare centered with the rotor lobes or a gear tooth when the lobes arecentered with a gear tooth space. To simplify this description, thegears will be described as having teeth centered with the rotor lobes,although the opposite could be true.

T better describe the invention, we can consider a three lobed pump,driven by interrneshing gears. The gears have teeth centered with eachrotor lobe, meeting the first requirement mentioned above. To center thelobe of one rotor with the valley of the other rotor, each gear musthave a tooth space angularly centered between the teeth aligned with thelobes. a three lobed rotor, the gears must have an odd number of teeth,as gears with an even number of teeth would provide a gear tooth midwaybetween the gear teeth aligned with the lobes. I

From this description, we have worked out the formula:

In other words, with where N equals the number of gear teeth, Mrepresents an arbitrary multiple or whole number, and L represents thenumber of lobes. Considering L to constitute 3 lobes, or 3, and M toconstitute an arbitrary multiple such as 5, the number of gear teeth (N)on the gears would be 5 times 6, plus or minus 3; or thirty, plus orminus three; equalling thirty-three or twenty-seven teeth. The sameforrnular can beused, regardless of the number of lobes on the pump.Obviously, the number of gear teeth on the gears must be sutficient tocause the gears to properly rotate.

A feature of the present invention resides in the fact that by locatingeach of the gears in the same relation to the rotors, the rotor shaftand gear assemblies are interchangeable and replaceable. In the past, ifit were necessary to replace a rotor, the lobes of the rotor and theteeth of the gear had to be properly timed to properly engage theremaining rotor. By aligning one of the teeth of the gear with each ofthe rotor lobes, this difficulty is obviated.

A feature of the present invention resides in a novel method ofaligning-the gears with the rotors.- Considering a three lobed rot-or, ajig is provided including a supporting surface on which the rotor issupported with two of the three lobes resting against the surface andwith the rotor shaft engaged in generally 'U-shaped fittings. The centerof the third lobe of the rotor is then on a plane through the axis ofthe rotor perpendicular to the supporting surfaces. The gear is mountedon the rotor shaft in the correct axial location and a tooth space onthe bottom of the gear is engaged with a tooth-shaped projection fixedon a part of the jig which is movable vertically but fixed from axial ortangential movement. A tooth on the top of the gear is, at the sametime, engaged in a notch in a locating arm, and which is movableparallel to the rotor support and cooperable with an indicating gage toshow if the gear has been properly centered relative to the rotor.

A further feature of the present invention resides in the provision of arotary pump in which the rotors are hydraulically balanced so that theends of the rotors are properly spaced from the end plate of the pump.

These and other objects and novel features of the present invention willbe more clearly and fully set forth in the following specification andclaims.

In the drawings forming a part of the specification:

FIGURE 1 is a vertical sectional view of a rotary pump showing thegeneral form of construction thereof.

FlGURvE 2 is a cross sectional view of the pump shown in FIGURE 1, theposition of the section being indicated by the line 22 of FEGURE 1.

FIGURE 3 diagrammatically illustrates the ends of a pair of rotors andthe gears on the rotor shafts, showing the'relations-hip between thelobes and the gears.

FIGURE 4 is a front elevational view of a jig employed to properlylocate the gears relative to the rotors.

FIGURE 5 is an end elevational View of the jig shown in FIGURE 4.

FIGURE 6 is an end elevational view of the jig, the view showing the endof the jig opposite that shown in FIGURE 5.

FIGURE 7 is a top plan view of the jig.

FEGUR'E 8 is an enlarged sectional detail of one of the pump rotorseals.

The rotary pump is indicated in general by the letter A. The pumpincludes a housing 10 having upper and lower arcuate portions 11 and 12which form intersecting rotor chambers 13 and 14 designed to accommodatethree-lobed rotors 15 and 16 respectively. The rotors 15' and 16 aremounted upon supporting shafts 17 and 19' respectively. As is usual inpumps of this type, the lobes of the rotors engage in the hollowportions between the lobes of the cooperating rotor to provide a minimumof leakage between the two rotors.

The housing 19 includes an inlet passage 29 and an outlet passage 21 incorrnnunication with the rotor chambers 13 and 14. The fluid beingpumped enters the inlet 29, is carried between the rotor lobes and thearcuate walls 11 and 12 and are discharged through the discharge passage21. Intermediate the ends of the rotors, a by-pass passage includingpassage portions 22 and 23 extend from the discharge 21 to the inlet 29.A cylindrical passage 24- termi-nating in a valve seat 25 connects thepassage portions 22 and 23. A pressure relief valve 26 is slidablysupported in the cylindrical passage 24 and includes a beveledperipheral flange 27 designed to engage against the valve seat 25.

The relief valve 26 includes an axial boss 29. A plug 39 is threadedinto an opening 31 in axial alignment with the valve 26. The plug 39 isprovided with an axial socket 32 in opposed relation to the valve 26. Aspring 33 is interposed between the base of the socket 32 and the flange27 of the valve 26. One end of the spring 33 is held in place by thewalls of the socket 32 while the other end of the spring encircles theboss 29 and is held position thereby.

The cylindrical skirt 34- of the valve 26 which slides Within thecylindrical passage 24 is notched as indicated at 35 to points inclosely spaced relation to the flange 27. Thus, when the relief valve ismoved due to abnormal pressures in the discharge passage, fluid may flowthrough the notches 35 in the hollow skirt 34 and into the passage 23connected to the pump inlet 20.

The ends of the housing are closed by end plates 36 and 37 which areproperly located by dowel pin 39 which extend through the end plates andinto the housing 10. Bearing supporting plates 49 and 4-1 are secured inface contact with the end plates 36 and 37 and are also located relativeto the end plates and the pump housing by suitable means such as by thedowel pins 39'. The bearing supporting plates 40 and 41 are apertured toaccommodate a pair of scaled bearings 42 and 43, the apertures beingindicated by the numerals 4-4- and 45. Seals 46 and 47 are supported ingrooves 49 and 50 respectively to seal the outer races of the bearings44- and 45 relative to the walls of the apertures 44 and 45'. The innerraces of the bearings may be sealed relative to the shaft 17.

The bearing supporting plates 41 and 41 are also provided with a secondpair of aligned apertures 51 and 52 which support sealed bearings 53 and54 which support the shaft 19. The outer races of the bearings 53 and 54are sealed relative to the apertures 51 and 52 by seals 55 and 56.

The ends of the rotors and 16 are provided with axial sockets 57 and 59respectively which include sealing units. As all of the seals are ofidentical form, the parts will be given similar identification numbers.Sealing rings 60 are slidably engaged in sockets 61 and 62. coaxial withthe shafts 17 and 19 in the end plates 36 and 37. The rings 69 aresealed relative to the walls of the socket by sealing rings 63 locatedin the externally grooved surface of the rings 69. A sealing ring 64-encircles the shaft 17 or 19 and is sealed relative thereto by a sealingring 65. The sealing ring 64 is urged into sealing relation with thering 69 by means of a spring 66 intenposed between the sealing ring 64and the base of the socket 57 or 59.

A cover plate 67 overlies the bearing supporting plate 40 and isconnected thereto by bolts 67' which extend through the cover plate,thnough the bearing supporting plate, the end plate, and into the pumphousing 10. A gear housing 69 including an end Wall 70; a peripheralwall 71 and an outwardly extending mounting flange 72 is connected tothe other end of the pump housing by means of bolts 73 which extendthrough the flange 72, through the bearing supporting plate 41, the endplate 37, and into the end of the housing 10. The end wall 70 isprovided with an opening 74 containing a seal 75 to accommodate theprojecting end 76 of the drive shaft 17.

Gears 77 and 79 are pinned to the shafts 17 and 19 respectively by pairsof taper pins 80 and 81 extending diametrically to apertures 82 and 83in axially spaced relation in the gears 77 and 79. The gears 77 and 79are provided with angularly spaced teeth 84 and 85, the number of whichis odd and divisible by three. In the particular arrangement illustratedin FIGURE 3 of the drawings, the gears 77 and 79 have 27 teeth. One ofthe teeth 84 is directly aligned with the center of each lobe of therotor 15, and one of the teeth 85 is exactly aligned with the center ofeach of the lobes of the rotor 16, and a gear tooth space is centeredbetween adjacent lobes. In view of this fact, the rotors are pre-timedso that upon insertion of the rotors into the pump so that a lobe of onerotor extends into the valley between the lobes of the other rotor, theteeth of the two gears will be automatically in mesh.

In the past, considerable time and eifort have been required to properlytime the rotors and gears so that the rotors will fit properly togetheras the gears rotate. A variation of a few thousandths of an inch in theposition of a key or keyway greatly affects the operation of the pump orrenders it inoperative. Any small variation in the lo-ction of a key orkeyway causes a considerably greater variation at the outer diameter ofthe gear or rotor. As a result, a time of from six to twenty hours hasbeen considered the usual time required to properly time the rotors andgears. In the present method which will be described, the same operationcan be completed in perhaps thirty minutes 'with accuracy virtuallyguaranteed.

In FIGURES 4 through 7 of the drawings a jig is disclosed which properlylocates the gears in pretimed relation to the lobes of the pump rotor.This jig includes a supporting base 89 on which are mounted flat toppedsurface blocks 90 and 91 having their upper surfaces on a common plane.The blocks 90 and 91 are held in place on the top of the base -89 by anysuitable means such as by the bolts 92 and 93. A shaft positioning block94 is secured on the base 89 by bolts 95 or other suitable means and agenerally similar shaft positioning block 96 is supported on the base 89at the opposite sides of the blocks 99 and 91 and held in place by bolts97 or other suitable means. As indicated in FIGURE 6 of the drawings,the upper end of each of the shaft positioning blocks 94 and 96 includesa central notch 99 having parallel shaft engaging surfaces 100 on theinner surface thereof. These surfaces 100 are accurately ground ormilled to fit portions of the rotor supporting shafts 17 and 19 and thesurfaces of the two blocks 94 and 96 are accurately aligned so as tohold the shaft engaged therein from any transverse movement.

The shaft guide block 96 is provided with a projecting ground surface101 against which the end of a rotor such as 15 may engage. The oppositeside of the block 96 is provided with a parallel flat surface 102 whichis engageable with a surface of a gear such as the gear 77. When therotor engages the surface 101 and the gear engages the surface 192, thetwo elements are properly axially spaced.

A clamping arm 103 is pivotally supported between a pair of ribs 104 onthe base 89 by means of a pivot 105. As indicated in FIGURE 5 of thedrawings, the clamping arm 1113 is provided with a bifurcated end 106through which the rotor shaft 17 may extend. Opposed ground surfaces 107properly locate the shaft relative to the clamping arm. As indicated inFIGURE 4 of the drawings, each side of the bifurcated end 106 isprovided with a laterally extending rounded projection 109 designed tobear against the gear '77. A clamping bolt 110 having a head 111 extendsthrough the clamping arm 103 and into the base 89 to clamp the roundedprojections 109 against the gear.

A means is provided for properly indexing the gears such as 77 relativeto the rotor 15. A plunger 112 is slidably supported in a vertical bore113 having its axis in a vertical plane through the axis of the rotorshaft 17. A head 114 is provided on the upper end of the plunger 113 andan upwardly extending lug 115 having a tapered upper extremity 116 whichis shaped similarly to the teeth of the gear 77 is mounted on top of thehead 114. The plunger 112 is raised and lowered by means of a drivingplunger 117 which is slidable in a vertical bore 119 coaxial with thebore 113. The driving plunger 117 is provided with a tapered notch 129which is designed to receive the frusto conical end 121 of a threadedshaft 122. The shaft 122 is threaded into the base 89and is providedwith a hand wheel 123 by means of which the threaded pin or shaft 122may be rotated. A pin 124 is slidably supported in an internallythreaded passage 125 which intercepts the axis of the plunger 117 andthe pin includes an end 126 which is slidably engaged in a slot 127extending longitudinally of the plunger 117. The pin 124 is held inplace by a set screw 129 and acts to limit the slidable movement of thepushing plunger 117 and also to prevent this plunger from rotation inthe bore 119.

An arm 130 is pivotally and slidably connected by a pivot 131 to abracket 138 mounted on the upper surface of the base 89. The arm 130 isprovided with a projection 132 including a notch 133 which is shapedsimilarly to the shape of the gear teeth. The pivot 131 extends througha slot 134 which is elongated longitudinally of the arm 131) so that thearm 13% may move horizontally within the limits of the slot 134. A gage135 is mounted upon a bracket 136 attached to the bracket 138 andincludes a plunger 137 which may be moved longitudinally of the sleeve139. The gage 135 indicates the position of the plunger 137 which abuttsagainst the end 140 of the arm 130 for a purpose which will bedescribed.

In operation, the rotor 15 which is fixedly mounted upon the shaft 17 isplaced upon the ground upper surfaces of the blocks 90 and 91 with twoof the three lobes of the rotor resting upon the horizontal uppersurfaces of these blocks. The shaft 17 is properly located between theground surfaces 160 of the shaft positioning block 94 and anintermediate portion of the shaft 17 is properly located between similarground surfaces of the shaft positioning block 96.- The end of the rotor15 is moved into contact with the locating surface 101. As is indicatedin FiGURE 6 of the drawings, a block 143 is mounted upon the base 39 toone side of the blocks which support the rotor 15. A stud 144 isthreaded into the top of the block 143 and is held in adjusted relationby means of a lock nut 145. The stud 144 is provided with an axiallyextending pin 1 16 which is engageable in a slot 147 in the undersurface of a clamping bar 149. The clamping bar 149 extends over thecenter line of the rotor shaft supports. A threaded stud 150 alsoextends into the block 143, the axes of the studs 144 and 151} being atright angles to the axis of the rotor shaft. A locking nut 151 holds thestud 159 at a proper elevation. A spring 152 encircles the stud 150above the nut 151 and supports a washer 153. The stud 150 extendsthrough a slotted aperture 154 in the clamping block 149 and a clampingnut 155 is provided on the end of the stud by means of which theclamping bar 149 may be clamped against the uppermost lobe of the rotor15, the pin 146 serving as a fulcrum point. The clamping bar 149 willclamp the rotor 15 firmly in place with the center of the uppermost lobeof the rotor on a vertical plane through the axis of the shaft.

The gear 77 is next manually moved against bearing surface 162 and theplunger 112 is raised by threading the threaded pin or shaft 122 intothe notch 120, the frusto conical end of the shaft acting as a camagainst the upper surface of the notch 120. When the wedge shapedprojection 116 on the plunger head 114 is properly engaged in'the spacebetween two of the gear teeth 84, one tooth at the top of the gear 77will be centered 6. on the vertical plane through the rotor shaft 17 andthrough the center of the uppermost lobe of the rotor.

When the gear is properly located, the arm 133 is pivoted down from thedotted line position illustrated in FIGURE 5 of the drawings to gearengaging position shown in full lines in this figure. The arm may movehorizontally within the limits of the groove 134, any such lateralmovement moving the plunger 137 of the gage 135. If the gear 77 isexactly in the proper location, the gage will so indicate, the gagebeing set to provide a predetermined zero reading when the arm 13%) isin proper position. If the gear 77 is not exactly in position, the toothengaged in the notch 133 will 'be too near or too far from the plunger137 and the gage will so indicate.

The clamping arm 1193 may then be tightened against the gear 77 to holdthe gear against the surface 162. If preferred, this clam-ping arm maybe tightened prior to the testing of the gear position by the arm 130.Once the clamping bar 149 and the clamping arm 103 are tightened, holesmay be drilled in axially spaced relation through the gear 77 and shaft17, these holes being indicated at 82 in FIGURE 1 of the drawings. Theholes 82 are reamed or otherwise cut to a tapered form and undercut atone end as indicated at 157 to accommodate the taper pin 80. Thisundercutting is done prior to the assembly procedure.

It will be seen that by selecting gears 77 and 79 with spaced peripheralteeth which are in multiples of three, three teeth of each gear will beproperly aligned with the center line of the three corresponding rotorlobes. By choosing these gears with an odd number of teeth, a toothspace is provided in alignment with each valley between the rotor lobes.Thus, in assembling the pump, the lobes of the rotors may be readilyarranged in properly timed relation through the meshing of the gears 77and 79. Obviously, the gear 79 is mounted upon the rotor shaft 19 inproperly timed relation with the rotor 16 in the manner described, therotor shafts being identical except for the difference in length. By theuse of the fixture, the gears may be pinned to the rotor shaft in arelatively few minutes, effectively accomplishing a result whichpreviously required from 6 to 20 hours of labor. Furthermore, should itever become necessary to replace a pump rotor, the new rotor may beinserted in place of the rotor removed and will automatically be inproper time as long as the proper teeth of the meshing gears areengaged.

While the apparatus has been described for use specifically for threelobed pumps, the same apparatus may be adapted for use in conjunctionwith pumps having more or less lobes. A two lobed pump must have gearsprovided with an even number of teeth which is not divisible by four,such as thirty, thirty-four, thirty-eight, etc. A four lobed pump wouldhave to have gears with an even number of teeth not divisible by eight,such as twenty-eight, thirty-six, fortyfour, etc.

The reason for this lies in the fact that if a gear tooth is in linewith each rotor lobe, a tooth space musttbe in line with each rotorvalley. If the letter L represents the nunrber of rotor lobes, theletter N represents the number of gear teeth, and M represents anarbitrary multiple or Whole number, we may determine the permis si blenumber of teeth on the gear by the formula:

in FIGURE 2 of the drawings a groove 159 is shown in the end plate 36forming an end wall of the pump. A similar groove is provided in the endplate 37. These grooves lead from the discharge side of the pump to theseal chambers or sockets 57 and 59, subjecting these chamberssubstantially to discharge pressure. If the rotor moves axially until itru bs against the end plate the clearance at that end will be reducedto'zer-o and the pressure in the seal chamlber will be substantiallyequal to discharge pressune. At the other end, however, the clearancebetween the rotor and the end plate will increase to a maximum, allowingliquid to bleed readily from the seal chamber to the suction side of thepump, reducing the pressure in the seal chamber to somewhat belowdischarge pressure. This results in an end thrust tending to move therotor to central position where it has equal clearance with each endplate.

In accordance with the patent statutes, we have described the principlesof construction and operation of our improvement in pre-timed threelobed rotary pump, and while we have endeavored to set forth the bestembodiment thereof, we desire to have it understood that charges may bemade within the scope of the following claims without departing from thespirit of our invention.

We claim:

1. A pro-timed three lobed rotary pump including a pair of three lobedrotors having valleys therelbetween, shafts supporting said rotors andprojecting from the ends thereof, means rotatably supporting said shaftswith said rotors interengaged, intemneshing gears on said shafts, saidgears having a similar odd number of teeth which is a multiple of three,one tooth of each gear having its center on a plane through the axis ofits shaft and through the center of a rotor lobe, and one tooth space ofeach gear being on a plane through the axis of the shaft and through thecenter of a rotor valley.

2. The structure of claim 1 and in which each gear is pinned to itssupporting shaft.

3. A rotor for a rotary pump including a .body having three equallyangularly spaced lobes, a rotor shaft supporting said body, and a gearon said shaft, said gear having an odd number of teeth which is amultiple of three, one tooth of said gear being centered with a planethrough the shaft axis and through the center of each lobe.

4. A pre-timed rotary pump including a pair of identical lobular rotorshaving three lobes, shafts supporting said rotors and projecting fromthe ends thereof, intermeshing gears on said shafts, said gears havingan odd number of teeth determined by the formula N (M X 2L) iL Where Nequals the number of gear teeth, M is an arbitrary Whole number and Lequals the number of lobes on each rotor, one tooth of each gear beingon a plane through the center of each lobe and through the axis of thesupporting shaft.

5. A method of producing pretimed three-lobed rotary pumps, the pumpsincluding a pair of interrneshing threelobed rotors, rotor shafts infixed relation to said rotors, and gears adapted to be secured to saidgears in intermeshing relation, the method consisting in selecting apair of gears having a similar odd number of teeth which is divisible bythree and attaching the gears to the shaft so that a plane through thecenter of one tooth of the gear and through the 'axis of the shaft is ina predetermined fixed relation to a plane, through the center of a rotorlobe and through the shaft and mounting each rotor so that the planethrough the center of one rotor lobe and its shaft intersects the centerof the space between the lobes of the other rotor and the axis of theother rotor shaft.

6. The method of claim 5 and including the step of pinning each gear toits respective shaft when the gear is in proper relation thereto.

7. A method of producing pretimed three-lobed pump havinginterchangeable rotors, the rotors having fixed axially extendingshafts, and gears on said shafts to drive the shafts in unison, themethod consisting in placing on the rotor shaft a gear having an oddnumber of teeth which is divisible by three, rotating the gear on theshaft until a plane through the center of one tooth of the gear andthrough the axis of the shaft is in a predetermined fixed relation to aplane through the center of a rotor lobe and through the shaft axes, therotors then being interchangeable and replaceable.

8. A pretimed rotary pump including a pair of identical lo'bula-r rotorshaving at least three lobes, shafts supporting said rotors andprojecting from the ends thereof, intermeshing gears on said shafts,said gears having a number of teeth determined by the formula N =(M 2L):L, where N equals the number of gear teeth, M is an arbitrary wholenumber and L equals the number of lobes on each rotor, one tooth of eachgear being centered on a plane through the shaft axis which is in fixedrelation to a plane through the shaft axis and through the center of oneof said lobes.

9. A method of producing pretim-ed rotary pumps having interchangeablerotors, the rotors having at least three lobes and having fixed axiallyextending shafts, and gears on said shafts to drive the shafts inunison, the method consisting of placing on each rotor shaft a gearhaving a number of teeth determined by the formula N=(MX2L):L where Nequals the number of gear teeth, M is an anbitrary Whole number and Lequals the number of lobes on each rotor, rotating the gear upon theshaft until one tooth of the gear is centered on a plane through theshaft axis which is in fixed relation to a plane through the shaft axisand through the center of one of said lobes, securing the gear in thisfixed relat on upon the shaft, and similarly attaching the other gear onthe shaft of the other rotor, the assembly of shaft, rotor, and gearbeing interchangeable with and replaceable by a like assembly.

References Cited in the file of this patent UNITED STATES PATENTS Re.2,369 Roots Oct. 2, 1866 30,157 Roots Sept. 25, 1860 345,969 HarroldJuly 20, 1886 515,212 Green Feb. 20, 1894 567,089 Garber Sept. 1, 1896659,619 Green Oct. 9, 1900 924,024 Wilkin June 8, 1909 1,407,496 StoreyFeb. 21, 1922 1,645,967 Patterson Oct. 18, 1927 1,854,260 Hofmann Apr.19, 1932 1,996,169 Padgett Apr. 2, 1935 2,247,454 Thomson July 1, 19412,281,157 Kanuch, et a1. Apr. 28, 1942 2,377,965 Rataiczak June 12, 19452,781,586 Eigenbrode Feb. 19, 1957

1. A PRE-TIMED THREE LOBED ROTARY PUMP INCLUDING A PAIR OF THREE LOBEDROTORS HAVING VALLEY THEREBETWEEN, SHAFT SUPPORTING SAID ROTORS ANDPROJECTING FROM THE ENDS THEREOF, MEANS ROTATABLY SUPPORTING SAID SHAFTSWITH SAID ROTORS INTERENGAGED, INTERMESHING GEARS ON SAID SHAFTS, SAIDGEARS HAVING A SIMILAR ODD NUMBER OF TEETH WHICH IS A MULTIPLE OF THREE,ONE TOOTH OF EACH GEAR HAVING ITS CENTER ON A PLANE THROUGH THE AXIS OFITS SHAFT AND THROUGH THE CENTER OF A ROTOR LOBE, AND ONE TOOTH SPACE OFEACH GEAR BEING ON A PLANE THROUGH THE AXIS OF THE SHAFT AND THROUGH THECENTER OF A ROTOR VALLEY.